U.S. patent number 9,931,669 [Application Number 15/106,373] was granted by the patent office on 2018-04-03 for coating method and coated article obtained by the same.
This patent grant is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The grantee listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Hisao Hayashi, Kazuyuki Tachi, Shuji Yomo.
United States Patent |
9,931,669 |
Yomo , et al. |
April 3, 2018 |
Coating method and coated article obtained by the same
Abstract
A coating method for forming a laminated coating film including:
preparing a thermosetting coating material as a lower layer-coating
material, an intermediate layer-coating material, and as an upper
layer-coating material; forming an uncured laminated coating film
by applying the lower layer-coating, the intermediate
layer-coating, and the upper layer-coating materials on the base
material using a wet-on-wet technique; and simultaneously curing
the lower layer-coating, the intermediate layer-coating, and upper
layer-coating materials by baking the uncured laminated coating
film. In the preparation step, the lower layer-coating, the
intermediate layer-coating, and the upper layer-coating materials
are selected so a sum of an absolute value of a difference in
shrinkage ratio between the lower layer-coating and the
intermediate layer-coating materials and an absolute value of a
difference in shrinkage ratio between the intermediate
layer-coating and the upper layer-coating materials at the late
stage of the baking is 3.0% or smaller.
Inventors: |
Yomo; Shuji (Nagakute,
JP), Tachi; Kazuyuki (Nagakute, JP),
Hayashi; Hisao (Nagakute, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi, Aichi |
N/A |
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI KAISHA
(Toyota-shi, Aichi, JP)
|
Family
ID: |
52345484 |
Appl.
No.: |
15/106,373 |
Filed: |
December 12, 2014 |
PCT
Filed: |
December 12, 2014 |
PCT No.: |
PCT/JP2014/083624 |
371(c)(1),(2),(4) Date: |
June 20, 2016 |
PCT
Pub. No.: |
WO2015/114989 |
PCT
Pub. Date: |
August 06, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170036244 A1 |
Feb 9, 2017 |
|
Foreign Application Priority Data
|
|
|
|
|
Jan 30, 2014 [JP] |
|
|
2014-015153 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B05D
3/0254 (20130101); B05D 7/582 (20130101); B05D
1/04 (20130101); B05D 7/5723 (20130101); B05D
7/572 (20130101); B05D 2508/00 (20130101); B05D
2502/005 (20130101) |
Current International
Class: |
B05D
7/00 (20060101); B05D 3/02 (20060101); B05D
1/04 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
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2 362 813 |
|
Sep 2011 |
|
EP |
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2 399 520 |
|
Sep 2004 |
|
GB |
|
2004-275966 |
|
Oct 2004 |
|
JP |
|
2007229671 |
|
Sep 2007 |
|
JP |
|
2007283271 |
|
Nov 2007 |
|
JP |
|
2010142712 |
|
Jul 2010 |
|
JP |
|
2010/038713 |
|
Apr 2010 |
|
WO |
|
Primary Examiner: Rodriguez; Michael P
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
The invention claimed is:
1. A coating method for forming a laminated coating film including
a lower layer formed on a base material, an intermediate layer
formed on the lower layer, and an upper layer formed on the
intermediate layer, the method comprising: a preparation step of
preparing a thermosetting coating material as a lower layer-coating
material for forming the lower layer, preparing a thermosetting
coating material as an intermediate layer-coating material for
forming the intermediate layer, and preparing a thermosetting
coating material as an upper layer-coating material for forming the
upper layer; a formation step of forming an uncured laminated
coating film by applying the lower layer-coating material, the
intermediate layer-coating material, and the upper layer-coating
material on the base material using a wet-on-wet technique; and a
baking step of simultaneously curing the lower layer-coating
material, the intermediate layer-coating material, and the upper
layer-coating material by subjecting the uncured laminated coating
film to a baking treatment, wherein in the preparation step, the
lower layer-coating material, the intermediate layer-coating
material, and the upper layer-coating material are selected so that
a sum of an absolute value of a difference in shrinkage ratio
between the lower layer-coating material and the intermediate
layer-coating material at a late stage of the baking in the baking
step and an absolute value of a difference in shrinkage ratio
between the intermediate layer-coating material and the upper
layer-coating material at the late stage of the baking in the
baking step is 3.0% or smaller, said late stage of the baking being
a period after a preliminary drying state in which water has been
removed by drying the coating film made of the coating material at
80.degree. C. for 3 hours and then in a vacuum at 60.degree. C. for
96 hours, up to a completion state of the baking in which the
coating film has been baked at 140.degree. C. for 30 minutes, each
of the upper layer-coating material, the intermediate layer-coating
material and the lower layer-coating material contains a
thermosetting resin and a curing agent, a combination of the
thermosetting resin and the curing agent in the upper layer-coating
material is a combination selected from the group consisting of a
combination of a hydroxy group-containing acrylic resin and an
isocyanate compound, a combination of a hydroxy group-containing
acrylic resin and an isocyanate resin, and a combination of a
hydroxy group and glycidyl group-containing acrylic resin and a
carboxyl group-containing acrylic resin, a combination of the
thermosetting resin and the curing agent in the intermediate
layer-coating material is a combination selected from the group
consisting of a combination of an acrylic resin and a melamine
resin, a combination of a polyester resin and a melamine resin, a
combination of an acrylic resin and a (block) isocyanate compound,
and a combination of a polyester resin and a (block) isocyanate
compound, and a combination of the thermosetting resin and the
curing agent in the lower layer-coating material is a combination
selected from the group consisting of a combination of an acrylic
resin and a melamine resin, a combination of a polyester resin and
a melamine resin, a combination of an acrylic resin and a (block)
isocyanate compound, and a combination of a polyester resin and a
(block) isocyanate compound.
2. The coating method according to claim 1, wherein the upper
layer-coating material has a shrinkage ratio in a range from 0 to
20% at the late stage of the baking in the baking step, the
intermediate layer-coating material has a shrinkage ratio in a
range from 0 to 20% at the late stage of the baking in the baking
step, and the lower layer-coating material has a shrinkage ratio in
a range from 0 to 20% at the late stage of the baking in the baking
step.
3. The coating method according to claim 1, wherein in the
preparation step, the intermediate layer-coating material and the
upper layer-coating material are selected so that the absolute
value of the difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step is 2.0% or smaller.
4. The coating method according to claim 1, wherein the upper
layer-coating material is a coating material containing no melamine
resin as a curing agent.
5. The coating method according to claim 1, wherein the upper
layer-coating material is a thermosetting coating material from
which no volatile product is formed in a curing reaction by a heat
treatment.
6. The coating method according to claim 1, wherein the upper
layer-coating material is a clear coating material, the
intermediate layer-coating material is a base coating material, and
the lower layer-coating material is an intermediate coating
material.
7. The coating method according to claim 1, wherein in the
preparation step, the lower layer-coating material, the
intermediate layer-coating material, and the upper layer-coating
material are selected so that a sum of an absolute value of a
difference in shrinkage ratio between the lower layer-coating
material and the intermediate layer-coating material at a late
stage of the baking in the baking step and an absolute value of a
difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step is 2.0% or smaller.
Description
TECHNICAL FIELD
The present invention relates to a coating method in which three
kinds of coating materials are applied using a wet-on-wet technique
and then simultaneously baked, and to a coated article obtained by
the same.
BACKGROUND ART
For forming a laminated coating film by a coating method in which
three kinds of coating materials are applied using a wet-on-wet
technique and then baked, there has been a conventionally used
method by which the laminated coating film as a whole is cured. In
this method, thermosetting coating materials for forming layers
constituting a laminated coating film are selected so that all the
layers can be cured at the same heating temperature after all the
coating materials are applied. However, the conventional coating
method has a problem that the obtained laminated coating film is
inferior in surface texture and gloss to that obtained by baking a
lower layer and then applying and baking coating materials for
forming an intermediate layer and an upper layer. In this
connection, various methods have been proposed to improve the
surface texture and the gloss of a laminated coating film.
For example, Japanese Unexamined Patent Application Publication No.
2004-275966 (PTL 1) discloses a method for forming a coating film,
the method comprising: a step of successively applying an
intermediate paint, abase paint, and a clear paint in a wet-on-wet
manner; and a heating step including both a low-temperature heating
stage (heating at a temperature which is 25 to 80% of a curing
temperature for a time which is 5 to 30% of a curing time) and a
high-temperature heating stage (heating at a temperature which
exceeds 80% and is not more than 120% of a curing temperature for a
time which is 30 to 130% of a curing time). However, in the case of
the conventional method for forming a multilayer coating film as
described in PTL 1 and an article coated by the method, the
appearance qualities, such as surface texture (smoothness) and
gloss of the laminated coating film are not necessarily sufficient,
and it is difficult to improve the surface texture and gloss to the
levels required for the appearance qualities of automobiles. In
this respect, coated articles having better appearance qualities
and better durability have been demanded for automobile steel
plates and the like, and further improvement of the wet-on-wet
coating method has been desired.
CITATION LIST
Patent Literature
[PTL 1] Japanese Unexamined Patent Application Publication No.
2004-275966
SUMMARY OF INVENTION
Technical Problem
The present invention has been made in view of the above-described
problems of the conventional technologies. An object of the present
invention is to provide a coating method which makes it possible to
obtain a laminated coating film having an upper layer in which
formation of surface unevenness is sufficiently suppressed, even
when three kinds of coating materials are applied using a
wet-on-wet technique and simultaneously baked to cure the layers
for the purpose of obtaining high durability and the like. Another
object of the present invention is to provide a coated article that
is obtained by the same and is very excellent in appearance
qualities.
Solution to Problem
The present inventers have conducted earnestly study to achieve the
above object, and consequently revealed the following fact in the
case where coating is conducted by applying three kinds of
thermosetting coating materials using a wet-on-wet technique and
simultaneously baking them. Specifically, a thermosetting coating
material is used as a lower layer-coating material for forming the
lower layer, a thermosetting coating material is used as an
intermediate layer-coating material for forming the intermediate
layer, and a thermosetting coating material is used as an upper
layer-coating material for forming the upper layer. Here, these
coating materials are selected so that a sum of an absolute value
of a difference in shrinkage ratio between the lower layer-coating
material and the intermediate layer-coating material at a late
stage of the baking in the baking step and an absolute value of a
difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step can be within a
specific range. Use of such coating materials makes it possible to
reduce the amount of transfer of the unevenness at the interface
between the upper layer and the intermediate layer to the upper
layer which has been cured with fluidity remarkably lowered and/or
makes it possible to reduce the unevenness at the interface between
the intermediate layer and the lower layer and the amount of
transfer of the unevenness to the upper layer which has been cured
with fluidity remarkably lowered. Accordingly, a laminated coating
film having further very excellent appearance qualities can be
obtained, even though the three kinds of coating materials are
applied using a wet-on-wet technique, and then simultaneously
baked. This finding has led to the completion of the present
invention.
The coating method of the present invention is a coating method for
forming a laminated coating film including a lower layer formed on
a base material, an intermediate layer formed on the lower layer,
and an upper layer formed on the intermediate layer, the method
comprising:
a preparation step of preparing a thermosetting coating material as
a lower layer-coating material for forming the lower layer,
preparing a thermosetting coating material as an intermediate
layer-coating material for forming the intermediate layer, and
preparing a thermosetting coating material as an upper
layer-coating material for forming the upper layer;
a formation step of forming an uncured laminated coating film by
applying the lower layer-coating material, the intermediate
layer-coating material, and the upper layer-coating material on the
base material using a wet-on-wet technique; and
a baking step of simultaneously curing the lower layer-coating
material, the intermediate layer-coating material, and the upper
layer-coating material by subjecting the uncured laminated coating
film to a baking treatment, wherein
in the preparation step, the lower layer-coating material, the
intermediate layer-coating material, and the upper layer-coating
material are selected so that a sum of an absolute value of a
difference in shrinkage ratio between the lower layer-coating
material and the intermediate layer-coating material at a late
stage of the baking in the baking step and an absolute value of a
difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step is 3.0% or smaller.
In the above-described coating method of the present invention, the
upper layer-coating material preferably has a shrinkage ratio in a
range from 0 to 20% at the late stage of the baking in the baking
step, the intermediate layer-coating material preferably has a
shrinkage ratio in a range from 0 to 20% at the late stage of the
baking in the baking step, and the lower layer-coating material
preferably has a shrinkage ratio in a range from 0 to 20% at the
late stage of the baking in the baking step.
In addition, in the above-described coating method of the present
invention, it is preferable that, in the preparation step, the
intermediate layer-coating material and the upper layer-coating
material be selected so that the absolute value of the difference
in shrinkage ratio between the intermediate layer-coating material
and the upper layer-coating material at the late stage of the
baking in the baking step is 2.0% or smaller.
Moreover, in the above-described coating method of the present
invention, the upper layer-coating material is preferably a coating
material containing no melamine resin as a curing agent.
Moreover, in the above-described coating method of the present
invention, the upper layer-coating material is preferably a
thermosetting coating material from which no volatile product is
formed in a curing reaction by a heat treatment.
Further, in the above-described coating method of the present
invention, each of the upper layer-coating material, the
intermediate layer-coating material and the lower layer-coating
material preferably contains a thermosetting resin and a curing
agent,
a combination of the thermosetting resin and the curing agent in
the upper layer-coating material is preferably a combination
selected from the group consisting of a combination of a hydroxy
group-containing acrylic resin and an isocyanate compound, a
combination of a hydroxy group-containing acrylic resin and an
isocyanate resin, and a combination of a hydroxy group and glycidyl
group-containing acrylic resin and a carboxyl group-containing
acrylic resin,
a combination of the thermosetting resin and the curing agent in
the intermediate layer-coating material is preferably a combination
selected from the group consisting of a combination of an acrylic
resin and a melamine resin, a combination of a polyester resin and
a melamine resin, a combination of an acrylic resin and a (block)
isocyanate compound, and a combination of a polyester resin and a
(block) isocyanate compound, and
a combination of the thermosetting resin and the curing agent in
the lower layer-coating material is preferably a combination
selected from the group consisting of a combination of an acrylic
resin and a melamine resin, a combination of a polyester resin and
a melamine resin, a combination of an acrylic resin and a (block)
isocyanate compound, and a combination of a polyester resin and a
(block) isocyanate compound.
Furthermore, in the above-described coating method of the present
invention, the upper layer-coating material is preferably a clear
coating material, the intermediate layer-coating material is
preferably a base coating material, and the lower layer-coating
material is preferably an intermediate coating material.
The coated article of the present invention comprises a laminated
coating film including a lower layer formed on a base material, an
intermediate layer formed on the lower layer, and an upper layer
formed on the intermediate layer, wherein the coated article is
obtained by the above coating method.
Note that, although it is not known exactly why the above-described
object is achieved by the present invention, the present inventors
speculate as follows. Specifically, in a conventional laminated
coating film formed using a wet-on-wet technique, thermosetting
coating materials are used for all layers including an upper layer,
and the laminated coating film is designed so that these layers are
simultaneously cured at the same heating temperature, or curing is
started sequentially from a lower layer. Accordingly, when the
thermosetting coating material for forming the upper layer is cured
by a heat treatment (baking treatment), curing of the thermosetting
coating material proceeds also in the lower layer of the upper
layer, and the layer already loses the fluidity. In each layer of
the laminated coating film, the thermosetting coating material is
cured by a condensation reaction or by an addition reaction after
the deblocking reaction of a curing agent. Accordingly, volatile
products formed in this condensation reaction or deblocking
reaction evaporate along with the residual solvents. This causes
the shrinkage of the laminated coating film, and thereby unevenness
is formed on the surface of the coating film. This surface
unevenness of the coating film is reduced by the flowing or the
like of the upper layer that keeps having sufficient fluidity.
However, the present inventors speculate that, when the fluidity of
the upper layer remarkably decreases because of the curing, the
unevenness on the surface of the base material or at each interface
between layers is transferred to the surface of the upper layer,
deteriorating the surface texture and the gloss of the laminated
coating film.
Also in a case where a thermosetting coating material containing an
isocyanate compound or an isocyanate resin as a curing agent is
used as an upper layer-coating material or the like, the upper
layer often loses the fluidity before the lower layer is cured,
because of the higher curing rate of the upper layer-coating
material. In this case, the curing of the lower layer proceeds,
after the upper layer is cured. Since the lower layer-coating
material used for conventional wet-on-wet application has poor
fluidity, the unevenness formed because of the shrinkage which
occurs when the curing of the lower layer proceeds is not
sufficiently reduced, and the unevenness on the surface of the base
material or at each interface between layers is transferred to the
surface of the upper layer. Presumably because of this, the surface
texture and the gloss of the laminated coating film
deteriorate.
To achieve the above-described object, the present inventors have
first focused on the fact that the appearance qualities such as
surface texture (smoothness) and gloss of the laminated coating
film are better, when the upper layer has less surface unevenness.
Then, the present inventors have found that the unevenness which
has an influence on the surface texture is attributable to the
non-uniformity of the amount of the coating material applied on the
surface of the base material during spraying and the amount of
shrinkage of the coating film during the drying step (including the
baking step) in the direction of the surface, while the unevenness
(corresponding to shorter wavelengths than those in the case of the
surface texture) which governs the gloss is attributable to the
non-uniformity of the amount of shrinkage of the coating film in
the drying step in the direction of the surface. In addition, of
the two types of the unevenness formed because of the
above-described two factors, the unevenness attributable to the
non-uniformity of the amount of the coating material applied on the
surface of the base material during the spraying in the direction
of the surface can be suppressed by improving the fineness of
particles of the coating material. However, this causes
deterioration in coating efficiency, which is an effective
utilization rate of the coating material. Hence, the improvement in
the fineness of particles of the coating material more than
necessary is not favorable in terms of costs and the like. For this
reason, it has been found that, to improve the appearance qualities
such as surface texture (smoothness) and gloss, the reduction of
the unevenness attributable to the non-uniformity of the amount of
shrinkage of the coating film in the direction of the surface in
the drying step is advantageous. Then, the present inventors have
found the following fact. Specifically, when a laminated coating
film is formed by applying a coating material for forming a lower
layer, a coating material for forming an intermediate layer, and a
coating material for forming an upper layer on a base material
using a wet-on-wet technique, and then simultaneously baking the
coating materials, the above-described unevenness is formed mainly
because the unevenness at the interface between the lower layer and
the intermediate layer and the unevenness at the interface between
the intermediate layer and the upper layer which are formed when
the lower layer-coating material, the intermediate layer-coating
material, and the upper layer-coating material are applied using a
wet-on-wet technique are transferred to the surface of the upper
layer because of the shrinkage of each layer, after the remarkable
lowering of the fluidity of the upper layer in the drying step.
Hence, if the sum of the absolute value of the difference in
shrinkage ratio between the lower layer and the intermediate layer
forming the interface at the late stage of the baking and the
absolute value of the difference in shrinkage ratio between the
intermediate layer and the upper layer forming the interface at the
late stage of the baking is small, the amount of the unevenness at
the interfaces transferred to the surface of the upper layer is
small.
In this respect, in a case where coating is carried out by applying
three kinds of thermosetting coating materials using a wet-on-wet
technique and simultaneously baking these materials, a
thermosetting coating material is used as the lower layer-coating
material for forming the lower layer, a thermosetting coating
material is used as the intermediate layer-coating material for
forming the intermediate layer, and a thermosetting coating
material is used as the upper layer-coating material for forming
the upper layer. Here, these coating materials are selected so that
the sum of the absolute value of the difference in shrinkage ratio
between the lower layer-coating material and the intermediate
layer-coating material at the late stage of the baking in the
baking step and the absolute value of the difference in shrinkage
ratio between the intermediate layer-coating material and the upper
layer-coating material at the late stage of the baking in the
baking step is 3.0% or smaller. Thus, the sum of the differences in
shrinkage ratio between the lower layer and the intermediate layer
and between the intermediate layer and the upper layer is
sufficiently reduced to be within a specific range. The present
inventors speculate that this makes it possible to sufficiently
reduce the unevenness at each interface and the amount of the
unevenness transferred to the upper layer, so that a laminated
coating film having further very excellent appearance qualities can
be obtained, even when three kinds of coating materials are applied
using a wet-on-wet technique and then simultaneously baked.
Advantageous Effects of Invention
According to the present invention, even when three kinds of
coating materials are applied using a wet-on-wet technique and
baked to cure all the layers for the purpose of obtaining high
durability and the like, a laminated coating film having an upper
layer in which formation of surface unevenness is sufficiently
suppressed can be obtained. Accordingly, the present invention
makes it possible to obtain a coated article having very excellent
appearance qualities such as surface texture (surface smoothness)
and gloss.
DESCRIPTION OF EMBODIMENTS
Hereinafter, the present invention will be described in details on
the basis of preferred embodiments thereof.
A coating method of the present invention is a coating method for
forming a laminated coating film including a lower layer formed on
a base material, an intermediate layer formed on the lower layer,
and an upper layer formed on the intermediate layer, the method
comprising:
a preparation step (Raw Coating Material Preparation Step) of
preparing a thermosetting coating material as a lower layer-coating
material for forming the lower layer, preparing a thermosetting
coating material as an intermediate layer-coating material for
forming the intermediate layer, and preparing a thermosetting
coating material as an upper layer-coating material for forming the
upper layer;
a formation step (Application Step) of forming an uncured laminated
coating film by applying the lower layer-coating material, the
intermediate layer-coating material, and the upper layer-coating
material on the base material using a wet-on-wet technique; and
a baking step (Baking Step) of simultaneously curing the lower
layer-coating material, the intermediate layer-coating material,
and the upper layer-coating material by subjecting the uncured
laminated coating film to a baking treatment, wherein
in the preparation step, the lower layer-coating material, the
intermediate layer-coating material, and the upper layer-coating
material are selected so that a sum of an absolute value of a
difference in shrinkage ratio between the lower layer-coating
material and the intermediate layer-coating material at a late
stage of the baking in the baking step and an absolute value of a
difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step is 3.0% or smaller.
(Raw Coating Material Preparation Step)
In the coating method of the present invention, first, a lower
layer-coating material for forming the lower layer, an intermediate
layer-coating material for forming the intermediate layer, and an
upper layer-coating material for forming the upper layer are
prepared.
A thermosetting coating material is used as the upper layer-coating
material according to the present invention. The thermosetting
coating material used as the upper layer-coating material only
needs to be one containing a thermosetting resin capable of forming
a coating film and a curing agent, and examples thereof include
thermosetting coating materials used as upper layer-coating
materials for ordinary baking finish. The form of the thermosetting
coating material for the upper layer may be any of solvent-based
form, water-based form, and powder form. A curing temperature of
the thermosetting coating material for the upper layer is not
particularly limited, and is generally 40 to 200.degree. C., and
preferably 80 to 160.degree. C. Note that, as the upper
layer-coating material, it is preferable to use a coating material
having a weight loss percentage of 0 to 20% by mass at the curing
temperature thereof. This leads to a tendency to minimize the
shrinkage of the coating film due to a heat treatment. Moreover,
from such a viewpoint, it is the most preferable to use a coating
material having a weight loss percentage of 0 to 10% by mass.
Note that, in the present invention, the curing temperature of a
coating material refers to a temperature at which the coating
material can be cured most efficiently in relation to other curing
conditions such as curing time, in the case where a target coating
material is applied to the base material, heat treatment is
performed, and the coating film is cured to be fixed on the base
material. In general, the curing temperature refers to a baking
temperature which is set (designed) for each coating material. In
the present invention, a value listed in its catalog can be
employed as this curing temperature (baking temperature).
Examples of the thermosetting resin that is contained in the upper
layer-coating material and is capable of forming a coating film
include hydroxy group-, glycidyl group-, or carboxyl
group-containing acrylic resins, polyester resins, alkyd resins,
epoxy resins, and urethane resins; however, the thermosetting resin
is not limited thereto. Preferable curing agents include isocyanate
compounds, block isocyanate compounds, isocyanate resins, and amino
compounds; however, the curing agent is not limited thereto. In
addition, one of these thermosetting resins may be used alone, or
two or more thereof may be used in combination. Also, one of these
curing agents may be used alone, or two or more thereof may be used
in combination.
Note that, preferably, the curing agent contained in the upper
layer-coating material does not contain any melamine resin. This
leads to a tendency to minimize the shrinkage of the coating film
due to a heat treatment.
In addition, the upper layer-coating material is preferably a
thermosetting coating material from which no volatile product is
formed in a curing reaction by a heat treatment. This leads to a
tendency to minimize the shrinkage of the coating film due to a
heat treatment.
Further, examples of combinations of the thermosetting resin and
the curing agent from which no volatile product is formed in the
curing reaction by the heat treatment include combinations of a
hydroxy group-containing acrylic resin with an isocyanate compound
and/or an isocyanate resin, and the like. In the present invention,
to obtain further excellent and high appearance qualities, a
thermosetting coating material to be cured by a heat treatment may
be applied on the upper layer of the laminated coating film cured
by being subjected to the heat treatment. This thermosetting
coating material is more preferably a coating material from which
substantially no volatile product is formed in the curing reaction
by the heat treatment.
Note that, in the present invention, the upper layer-coating
material is prepared by selecting a combination of the
thermosetting resin and the curing agent to be contained in the
upper layer-coating material so that the sum of the absolute value
of the difference in shrinkage ratio between the lower
layer-coating material and the intermediate layer-coating material
at the late stage of the baking in the baking step and the absolute
value of the difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step is within the
above-described range. The combination of the thermosetting resin
and the curing agent is preferably a combination of a hydroxy
group-containing acrylic resin and an isocyanate compound, a
combination of a hydroxy group-containing acrylic resin and an
isocyanate resin, or a combination of a hydroxy group and glycidyl
group-containing acrylic resin and a carboxyl group-containing
acrylic resin.
Moreover, the upper layer-coating material is preferably a so
called "clear coating material" for forming a clear coating film
(clear layer) used for automobile coating material and coating. The
clear coating material may be, for example, one containing a
thermosetting resin, an organic solvent, and if necessary, an
ultraviolet absorber or the like and being capable of forming a
transparent coating film. Examples of the thermosetting resin
include those containing a resin, such as an acrylic resin, a
polyester resin, an alkyd resin, a fluororesin, a urethane resin,
or a silicon-containing resin, having a cross-linkable functional
group such as a hydroxy group, a carboxyl group, a silanol group,
or an epoxy group and a cross-linking agent which is capable of
reacting with the cross-linkable functional group, such as a urea
resin, a (block) polyisocyanate compound, an epoxy resin compound
or resin, a carboxyl group-containing compound or resin, an acid
anhydride, or an alkoxysilane group-containing compound or
resin.
In addition, the upper layer-coating material of the present
invention may contain conventionally known coloring pigments,
effect or luster pigments, and the like within a conventionally
known scope, when needed. Meanwhile, in order to adjust various
properties, various additives such as a viscosity controlling
agent, a surface conditioner, a thickening agent, an antioxidant,
an ultraviolet absorber, and a defoamer may be blended within a
conventionally known scope.
As the intermediate layer-coating material according to the present
invention, a thermosetting coating material is used. The
thermosetting coating material used as the intermediate
layer-coating material only needs to contain a thermosetting resin
capable of forming a coating film and a curing agent, and examples
thereof include thermosetting coating materials used as
intermediate layer-coating materials for ordinary baking finish.
The form of the thermosetting coating material for the intermediate
layer may be any of solvent-based form, water-based form, and
powder form. The curing temperature of the thermosetting coating
material for the intermediate layer is not particularly limited,
and is generally 40 to 200.degree. C., and preferably 80 to
160.degree. C.
Examples of the thermosetting resin that is capable of forming a
coating film contained in the intermediate layer-coating material
include acrylic resins, polyester resins, alkyd resins, epoxy
resins, and urethane resins; however, the thermosetting resin is
not limited thereto. Examples of the curing agent include amino
compounds, amino resins, isocyanate compounds, block isocyanate
compounds, and isocyanate resins; however, the curing agent is not
limited thereto. In addition, one of these thermosetting resins may
be used alone, or two or more thereof may be used in combination.
Also, one of these curing agents may be used alone, or two or more
thereof may be used in combination.
Note that, in the present invention, the intermediate layer-coating
material is prepared by selecting a combination of the
thermosetting resin and the curing agent contained in the
intermediate layer-coating material, so that the sum of the
absolute value of the difference in shrinkage ratio between the
lower layer-coating material and the intermediate layer-coating
material at the late stage of the baking in the baking step and the
absolute value of the difference in shrinkage ratio between the
intermediate layer-coating material and the upper layer-coating
material at the late stage of the baking in the baking step is
within the above-described range. The combination of the
thermosetting resin and the curing agent is preferably a
combination of an acrylic resin and a melamine resin, a combination
of a polyester resin and a melamine resin, a combination of an
acrylic resin and a (block) isocyanate compound, or a combination
of a polyester resin and a (block) isocyanate compound.
Moreover, the intermediate layer-coating material is preferably a
so called "base coating material" for forming a base coating film
(base layer) used for automobile coating material and coating. For
example, known solvent-based colored base coating materials and
water-based colored base coating material are preferably used.
Examples of the water-based colored base coating materials include
those containing a pigment, a water-soluble or dispersible resin,
across-linking agent, if necessary, and water as a solvent. The
water-soluble or dispersible resin may be, for example, a resin
having a hydrophilic group such as a carboxyl group and a
cross-linkable functional group such as a hydroxy group in a single
molecule, and specific examples thereof include acrylic resins,
polyester resins, polyurethane resins, and the like. Meanwhile,
examples of the cross-linking agent include hydrophobic or
hydrophilic alkyl ether melamine resins, block isocyanate
compounds, and the like. Meanwhile, examples of the solvent-based
colored base coating materials include those containing a pigment,
a resin as described above, a cross-linking agent, if necessary,
and a solvent.
In addition, the intermediate layer-coating material of the present
invention may contain conventionally known coloring pigments,
effect or luster pigments, and the like within a conventionally
known scope, when needed. Meanwhile, in order to adjust various
properties, various additives such as a viscosity controlling
agent, a surface conditioner, a thickening agent, an antioxidant,
an ultraviolet absorber, and a defoamer may be blended within a
conventionally known scope.
As the lower layer-coating material according to the present
invention, a thermosetting coating material is used. The
thermosetting coating material used as the lower layer-coating
material only needs to contain a thermosetting resin capable of
forming a coating film and a curing agent, and examples thereof
include thermosetting coating materials used as lower layer-coating
materials for ordinary baking finish. The form of the thermosetting
coating material for the lower layer may be any of solvent-based
form, water-based form, and powder form. The curing temperature of
the thermosetting coating material for the lower layer is not
particularly limited, and is generally 40 to 200.degree. C., and
preferably 80 to 160.degree. C.
Examples of the thermosetting resin that is capable of forming a
coating film contained in the lower layer-coating material include
acrylic resins, polyester resins, alkyd resins, epoxy resins, and
urethane resins; however, the thermosetting resin is not limited
thereto. Examples of the curing agent include amino compounds,
amino resins, isocyanate compounds, block isocyanate compounds, and
isocyanate resins; however, the curing agent is not limited
thereto. In addition, one of these thermosetting resins may be used
alone, or two or more thereof may be used in combination. Also, one
of these curing agents may be used alone, or two or more thereof
may be used in combination.
Note that, in the present invention, the lower layer-coating
material is prepared by selecting a combination of the
thermosetting resin and the curing agent to be contained in the
lower layer-coating material, so that the sum of the absolute value
of the difference in shrinkage ratio between the lower
layer-coating material and the intermediate layer-coating material
at the late stage of the baking in the baking step and the absolute
value of the difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step is within the
above-described range. The combination of the thermosetting resin
and the curing agent is preferably a combination of an acrylic
resin and a melamine resin, a combination of a polyester resin and
a melamine resin, a combination of an acrylic resin and a (block)
isocyanate compound, or a combination of a polyester resin and a
(block) isocyanate compound.
Moreover, the lower layer-coating material is preferably a
so-called "intermediate coating material" for forming an
intermediate coating film (intermediate coat layer) used for
automobile coating material and coating. For example, a
thermosetting resin composition comprising a base resin and a
cross-linking agent is suitably used. Examples of the base resin
include acrylic resins, polyester resins, alkyd resins, and the
like which have two or more cross-linkable functional groups such
as hydroxy groups, epoxy groups, isocyanate groups, or carboxyl
groups in a single molecule. Meanwhile, examples of the
cross-linking agent include amino resins such as melamine resins
and urea resins, optionally blocked polyisocyanate compounds,
carboxyl group-containing compounds, and the like.
In addition, the lower layer-coating material of the present
invention may contain conventionally known coloring pigments,
effect or luster pigments, and the like within a conventionally
known scope, when needed. Meanwhile, in order to adjust various
properties, various additives such as a viscosity controlling
agent, a surface conditioner, a thickening agent, an antioxidant,
an ultraviolet absorber, and a defoamer may be blended within a
conventionally known scope.
Note that, in the raw coating material preparation step of the
present invention, it is preferable to prepare the lower
layer-coating material and the upper layer-coating material so that
the absolute value of the difference in shrinkage ratio between the
lower layer-coating material and the upper layer-coating material
is 2.0% or smaller at the late stage of the baking in the step of
simultaneously curing the lower layer-coating material and the
upper layer-coating material by subjecting the uncured laminated
coating film to a baking treatment after the uncured laminated
coating film is formed by applying the lower layer-coating material
and the upper layer-coating material on the base material using a
wet-on-wet technique.
Note that, in the raw coating material preparation step of the
present invention, it is necessary to select the lower
layer-coating material, the intermediate layer-coating material,
and the upper layer-coating material so that the sum of the
absolute value of the difference in shrinkage ratio between the
lower layer-coating material and the intermediate layer-coating
material at the late stage of the baking in the baking step and the
absolute value of the difference in shrinkage ratio between the
intermediate layer-coating material and the upper layer-coating
material at the late stage of the baking in the baking step is 3.0%
or smaller.
Regarding the upper layer-coating material, the intermediate
layer-coating material and the lower layer-coating material, the
upper layer-coating material preferably has a shrinkage ratio in a
range from 0 to 20% at the late stage of the baking in the baking
step, the intermediate layer-coating material preferably has a
shrinkage ratio in a range from 0 to 20% at the late stage of the
baking in the baking step, and the lower layer-coating material
preferably has a shrinkage ratio in a range from 0 to 20% at the
late stage of the baking in the baking step. This leads to a
tendency that a laminated coating film having an upper layer with
less surface unevenness can be obtained, and consequently it tends
to be possible to obtain a coated article having very excellent
appearance qualities such as surface texture (surface smoothness)
and gloss.
Regarding the upper layer-coating material, the intermediate
layer-coating material, and the lower layer-coating material, the
upper layer-coating material is preferably a coating material of an
acid-epoxy curing system, an isocyanate-curing system, or a
melamine-curing system, the intermediate layer-coating material is
preferably a coating material of a melamine-curing system or an
isocyanate-curing system, and the lower layer-coating material is
preferably a coating material of a melamine-curing system or
isocyanate-curing system.
Further, the combination of the upper layer-coating material, the
intermediate layer-coating material, and the lower layer-coating
material is more preferably such that the upper layer-coating
material/intermediate layer-coating material/lower layer-coating
material is acid-epoxy curing system/melamine-curing
system/melamine-curing system, acid-epoxy curing
system/melamine-curing system/isocyanate-curing system, acid-epoxy
curing system/isocyanate-curing system/melamine-curing system,
acid-epoxy curing system/isocyanate-curing system/isocyanate-curing
system, isocyanate-curing system/melamine-curing
system/melamine-curing system, isocyanate-curing
system/melamine-curing system/isocyanate-curing system,
isocyanate-curing system/isocyanate-curing system/melamine-curing
system, or isocyanate-curing system/isocyanate-curing
system/isocyanate-curing system.
(Application Step)
Next, in the coating method of the present invention, an uncured
laminated coating film is formed by applying, on the base material,
the lower layer-coating material, the intermediate layer-coating
material, and the upper layer-coating material prepared in the raw
coating material preparation step using a wet-on-wet technique.
The base material according to the present invention is not
particularly limited, and examples thereof include metal materials
such as iron, aluminum, brass, copper, stainless steel, tinplate,
zinc-plated steel, and alloyed-zinc (Zn--Al, Zn--Ni, Zn--Fe, or the
like) plated steel; resins such as polyethylene resin,
polypropylene resin, acrylonitrile-butadiene-styrene (ABS) resin,
polyamide resin, acrylic resin, vinylidene chloride resin,
polycarbonate resin, polyurethane resin, and epoxy resin; various
plastic materials such as FRPs; inorganic material s such as glass,
cement, and concrete; wood; fiber materials (paper, fabrics, and
the like); foamed materials; and the like. Of these materials,
metal materials and plastic materials are preferable, and metal
materials are particularly preferable. The present invention is
preferably applied especially to automobile steel plates which are
required to have high appearance qualities. The surfaces of these
base materials may be subjected, in advance, to an
electrodeposition treatment, electrodeposition and intermediate
coating treatments, or the like.
In the application step according to the present invention, first,
the lower layer-coating material is applied on the basematerial,
and, if necessary, the solvent and the like are evaporated by
drying or the like, to form an uncured lower layer. Subsequently,
the intermediate layer-coating material is applied on the uncured
lower layer, and, if necessary, the solvent and the like are
evaporated by drying or the like, to form an uncured intermediate
layer. Next, the upper layer-coating material is applied on the
uncured intermediate layer, and, if necessary, the solvent and the
like are evaporated by drying or the like, to form an uncured upper
layer. Examples of methods for applying the lower layer-coating
material, the intermediate layer-coating material, and the upper
layer-coating material include conventionally known methods such as
air spray coating, air electrostatic spray coating, and rotary
atomizing electrostatic coating.
Note that the film thickness of the lower layer can be
appropriately set in accordance with a desired application. For
example, the film thickness after the heat treatment is preferably
5 to 50 .mu.m, and more preferably 10 to 40 .mu.m. If the film
thickness of the lower layer is less than the lower limit, it tends
to be difficult to obtain a uniform coating film as the lower
layer. On the other hand, if the film thickness exceeds the upper
limit, there are tendencies that the lower layer absorbs a large
amount of solvent and the like contained in the coating film as the
upper layer, and that the evaporation of the solvent contained in
the lower layer itself is prevented and thereby the appearance
qualities of the laminated coating film are deteriorated.
The film thickness of the intermediate layer can also be
appropriately set in accordance with a desired application. For
example, the film thickness after the heat treatment is preferably
5 to 50 .mu.m, and more preferably 10 to 40 .mu.m. If the film
thickness of the intermediate layer is less than the lower limit,
it tends to be difficult to obtain a coating film having a uniform
intermediate layer. On the other hand, if the film thickness
exceeds the upper limit, there are tendencies that the intermediate
layer absorbs a large amount of solvent and the like contained in
the coating film as the upper layer, and that the evaporation of
the solvent contained in the layer itself is also prevented and
thereby the appearance qualities of the laminated coating film are
deteriorated.
Further, the film thickness of the upper layer can be appropriately
set in accordance with a desired application. For example, the film
thickness after the heat treatment is preferably 15 to 60 .mu.m,
and more preferably 20 to 50 .mu.m. If the film thickness of the
upper layer is less than the lower limit, the fluidity is
insufficient and thereby the appearance qualities of the laminated
coating film tend to be deteriorated. On the other hand, if the
film thickness exceeds the upper limit, the fluidity is excessively
high, and thereby defects such as sagging tend to occur in a case
where the coating is performed in a vertical direction.
(Baking Step)
Next, in the coating method of the present invention, the lower
layer-coating material, the intermediate layer-coating material,
and the upper layer-coating material are simultaneously cured by
subjecting the uncured laminated coating film obtained in the
application step to a baking treatment (heat treatment).
Note that, in the baking step, it is necessary that the sum of the
absolute value of the difference in shrinkage ratio between the
lower layer-coating material and the intermediate layer-coating
material at the late stage of the baking in the baking step and the
absolute value of the difference in shrinkage ratio between the
intermediate layer-coating material and the upper layer-coating
material at the late stage of the baking in the baking step be 3.0%
or smaller. A conventional laminated coating film obtained using a
wet-on-wet technique cannot achieve the sum of the absolute value
of the difference in shrinkage ratio being 3.0% or smaller, unless
the combination of the upper layer, the intermediate layer, and the
lower layer is deliberately selected. When the sum of the absolute
values of the differences in shrinkage ratio exceeds 3.0%, it is
not possible to sufficiently reduce the amount of transfer of the
unevenness at the interface between the upper layer and the
intermediate layer and/or between the intermediate layer and the
lower layer to the upper layer which has been cured with fluidity
remarkably lowered. As a result, a laminated coating film having
excellent appearance qualities cannot be obtained, when the three
kinds of coating materials are applied using a wet-on-wet technique
and then simultaneously baked. Moreover, the sum of the absolute
value of the difference in shrinkage ratio between the lower
layer-coating material and the intermediate layer-coating material
at the late stage of the baking in the baking step and the absolute
value of the difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step is more preferably 2.0%
or smaller, and particularly preferably 1.0% or smaller.
Consequently, it tends to be possible to obtain a laminated coating
film having an upper layer in which formation of surface unevenness
is sufficiently suppressed, even when three kinds of coating
materials are applied using a wet-on-wet technique and baked to
cure all the layers for the purpose of obtaining high durability
and the like. Thus, it tends to be possible to obtain a coated
article having further very excellent appearance qualities such as
surface texture (surface smoothness) and gloss.
In addition, in the baking step, the absolute value of the
difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking is preferably 2.0% or smaller, more
preferably 1.0% or smaller, and particularly preferably 0.5% or
smaller. Consequently, it tends to be possible to obtain a
laminated coating film having an upper layer with further
sufficiently less surface unevenness, even when three kinds of
coating materials are applied using a wet-on-wet technique and
baked to cure all the layers for the purpose of obtaining high
durability and the like. Thus, it tends to be possible to obtain a
coated article having further very excel lent appearance qualities
such as surface texture (surface smoothness) and gloss.
<Method for Calculating Difference in Shrinkage Ratio>
In the present invention, the "shrinkage ratio" is defined as the
shrinkage ratio measured by the following method. Specifically,
since it is difficult to measure the shrinkage ratio of each layer
in the state of the laminated coating film and after the remarkable
lowering of the fluidity of the upper layer, the shrinkage ratios
(.omega.') of the upper layer coating film, the intermediate layer
coating film, and the lower layer coating film at the late stage of
the baking are measured in the state of single-layer films of these
coating materials. Here, the shrinkage ratios (.omega.') are
attributable to the evaporation of volatile products in the curing
reaction and residual solvents such as high-boiling point solvents
at the late stage of the baking. Then, from the shrinkage ratio of
the upper layer-coating material, the shrinkage ratio of the
intermediate layer-coating material, and the shrinkage ratio of the
upper layer-coating material, the "absolute value of a difference
in shrinkage ratio" (|.DELTA..omega..sub.A'|) between the lower
layer-coating material and the intermediate layer-coating material
at the late stage of the baking in the baking step and the
"absolute value of a difference in shrinkage ratio"
(|.DELTA..omega..sub.B'|) between the intermediate layer-coating
material and the upper layer-coating material at the late stage of
the baking in the baking step are determined. Further, a
calculation is carried out to determine the sum (|.DELTA..omega.'|)
of the absolute value (|.DELTA..omega..sub.A'|) of the difference
in shrinkage ratio between the lower layer-coating material and the
intermediate layer-coating material at the late stage of the baking
in the baking step and the absolute value (|.DELTA..omega..sub.B'|)
of the difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step.
Note that the "shrinkage ratios" and the "absolute value of a
difference in shrinkage ratio" are calculated by the following
method on the basis of the weight (g) of the coating film
immediately before the baking step (at the start of the baking
step).
First, the upper layer-coating material (A), the intermediate
layer-coating material (M), and the lower layer-coating material
(B) are each applied on a sample base material (for example,
stainless steel), so that the layer can have a target film
thickness in a laminated coating film after the heat treatment.
Then, each material is preliminarily dried (for example, dried at
60.degree. C. for 96 hours), and then cured by heating at
140.degree. C. for 30 minutes. Then, the weight is measured. The
shrinkage ratio .omega.' is calculated on the basis of the formula
(1): .omega.'=100(Y-Z)/(Z-X) (1), (in the formula, .omega.'
represents the shrinkage ratio (%) mainly attributable to volatile
products, X represents the weight (g) of the sample base material,
Y represents the weight (g) of the sample base material and the
coating film after the preliminary drying, and Z represents the
weight (g) of the sample base material and the coating film after
the curing by heating at 140.degree. C. for 30 minutes).
Note that the shrinkage ratio (.omega.') of each of the upper
layer-coating material (U), the intermediate layer-coating material
(M), and the lower layer-coating material (L) is calculated by the
corresponding one of the formulae (1-1), (1-2), and (1-3):
.omega..sub.U'=100(Y.sub.U-Z.sub.U)/(Z.sub.U-X.sub.U) (1-1),
.omega..sub.M'=100(Y.sub.M-Z.sub.M)/(Z.sub.M-X.sub.M) (1-2), and
.omega..sub.L'=100(Y.sub.L-Z.sub.L)/(Z.sub.L-X.sub.L) (1-3).
Next, the absolute value (|.DELTA..omega..sub.A'|) of the
difference between the shrinkage ratio of the lower layer-coating
film and the shrinkage ratio of the intermediate layer-coating film
is calculated by the formula (2-1), and the absolute value
(|.DELTA..omega..sub.B') of the difference between the shrinkage
ratio of the intermediate layer-coating film and the shrinkage
ratio of the upper layer-coating film is calculated by the formula
(2-2): |.DELTA..omega..sub.A'|=|.omega..sub.L'-.omega..sub.M'|
(2-1), and |.DELTA..omega..sub.B'|=|.omega..sub.M'-.omega..sub.U'|
(2-2).
Subsequently, the sum (|.DELTA..omega.'|) of the absolute value
(|.DELTA..omega..sub.A'|) of the difference in shrinkage ratio
between the lower layer-coating material and the intermediate
layer-coating material at the late stage of the baking in the
baking step and the absolute value (|.DELTA..omega..sub.B'|) of the
difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step is calculated by the
formula (3):
|.DELTA..omega.'|=|.DELTA..omega.'.sub.A|+|.DELTA..omega.'.sub.B|
(3).
In the present invention, the "late stage of the baking" refers to
the period after the preliminary drying up to the completion of the
baking. The preliminary drying refers to a state in which water has
been removed by drying the coating film at 80.degree. C. for 3
hours and then in a vacuum at 60.degree. C. for 96 hours. The
completion of the baking refers to a state in which the coating
film has been baked at 140.degree. C. for 30 minutes.
Note that, in the baking step of the present invention, the baking
treatment (heat treatment) preferably includes a heat treatment at
or above the temperature at which at least the upper layer is
cured, for example, at or above [the curing temperature of the
upper layer-coating material -20.degree. C.]. Meanwhile, the
heating time is preferably 50% or more and 150% or less of the
curing time of the upper layer-coating material.
In addition, in the coating method of the present invention, to
stabilize the coating film applied using a wet-on-wet technique and
remaining in the uncured state, the coating film is preferably
allowed to stand (flashed) at room temperature before the baking
treatment (heat treatment). The flashing time is set to 1 to 20
minutes, in general.
Moreover, in the present invention, to obtain a coated article
having appearance with higher quality, it is preferable to forma
surface layer by further applying one kind or more of coating
materials on the upper layer of the coated article obtained by the
coating method and subjecting the coated article to a heat
treatment. As the coating material, those listed as the examples of
the upper layer-coating material can be used. In addition, examples
of the method for applying the coating material include
conventionally known methods such as air spray coating, air
electrostatic spray coating, and rotary atomizing electrostatic
coating.
A coated article of the present invention is produced by the
above-described coating method of the present invention. In the
coated article of the present invention, the laminated coating film
has surface unevenness which is sufficiently less than that of a
laminated coating film produced using a conventional wet-on-wet
technique, and the coated article of the present invention has very
excellent appearance qualities. In addition, the laminated coating
film is formed by applying the coating material for forming the
lower layer and the coating material for forming the upper layer on
the base material using a wet-on-wet technique, and then
simultaneously baking the materials. Thus, energy saving, cost
reduction, and shortening of the process can be achieved to a great
extent. In addition, when a water-based coating material using
water as the major solvent is employed, emission of volatile
organic compounds (VOC) can be reduced. Such a coated article is
useful especially for vehicle bodies and parts for automobiles such
as passenger cars, trucks, buses, and motorcycles.
EXAMPLES
Hereinafter, the present invention will be described more
specifically on the basis of Examples and Comparative Examples.
However, the present invention is not limited to the following
Examples. Note that the shrinkage ratio of the lower layer-coating
material, the shrinkage ratio of the intermediate layer-coating
material, the shrinkage ratio of the upper layer-coating material
at the late stage of the baking in the baking step, the absolute
value of the difference in shrinkage ratio between the lower
layer-coating material and the intermediate layer-coating material,
the absolute value of the difference in shrinkage ratio between the
intermediate layer-coating material and the upper layer-coating
material, and the sum of the absolute value of the difference in
shrinkage ratio between the lower layer-coating material and the
intermediate layer-coating material at the late stage of the baking
in the baking step and the absolute value of the difference in
shrinkage ratio between the intermediate layer-coating material and
the upper layer-coating material at the late stage of the baking in
the baking step were calculated by the following methods.
<Calculation of Shrinkage Ratios of Coating Materials, Absolute
Values of Differences in Shrinkage Ratio, and Sum of Absolute
Values at Late Stage of Baking in Baking Step>
First, each of an upper layer-coating material (U), an intermediate
layer-coating material (M), and a lower layer-coating material (L)
was applied by air spraying on weighed stainless steel foil [15
cm.times.3 cm.times.50 .mu.m], so that the film obtained after the
heat treatment could have a target film thickness in a laminated
coating film. The coated foil was dried at 80.degree. C. for 3
hours, and in a vacuum (10.sup.-2 Torr or below) at 60.degree. C.
for 96 hours, and then weighed. Further, the dried coated foil was
baked at 140.degree. C. for 30 minutes, and then weighed. The
shrinkage ratio .omega.' was calculated on the basis of the formula
(11): .omega.'=100(Y-Z)/(Z-X) (11), (in the formula, .omega.'
represents the shrinkage ratio (%) mainly attributable to volatile
products, X represents the weight (g) of the stainless steel foil,
Y represents the weight (g) of the stainless steel foil and the
coating film after drying at 60.degree. C. for 96 hours in a
vacuum, and Z represents the weight (g) of the stainless steel foil
and the coating film after baking at 140.degree. C. for 30
minutes).
Note that the shrinkage ratio (.omega.') of each of the upper
layer-coating material (U), the intermediate layer-coating material
(M), and the lower layer-coating material (L) was as shown in the
corresponding one of the formulae (11-1), (11-2), and (11-3):
.omega..sub.U'=100(Y.sub.U-Z.sub.U)/(Z.sub.U-X.sub.U) (11-1),
.omega..sub.M'=100(Y.sub.M-Z.sub.M)/(Z.sub.M-X.sub.M) (11-2), and
.omega..sub.L'=100(Y.sub.L-Z.sub.L)/(Z.sub.L-X.sub.L) (11-3).
Next, the absolute value (|.DELTA..omega..sub.A'|) of the
difference between the shrinkage ratio of the lower layer-coating
film and the shrinkage ratio of the intermediate layer-coating film
was calculated by the formula (12-1), and the absolute value
(|.DELTA..omega..sub.B'|) of the difference between the shrinkage
ratio of the intermediate layer-coating film and the shrinkage
ratio of the upper layer-coating film was calculated by the formula
(12-2): |.DELTA..omega..sub.A'|=|.omega..sub.L'-.omega..sub.M'|
(12-1), and |.DELTA..omega..sub.B'|=|.omega..sub.M'-.omega..sub.U'|
(12-2).
Subsequently, the sum (|.DELTA..omega.'|) of the absolute value
(|.DELTA..omega..sub.A'|) of the difference in shrinkage ratio
between the lower layer-coating material and the intermediate
layer-coating material at the late stage of the baking in the
baking step and the absolute value (|.DELTA..omega..sub.B'|) of the
difference in shrinkage ratio between the intermediate
layer-coating material and the upper layer-coating material at the
late stage of the baking in the baking step was calculated by the
formula (13):
|.DELTA..omega.'|=|.DELTA..omega.'.sub.A|+|.DELTA..omega.'.sub.B|
(13).
(Synthesis Example 1) Preparation of Acrylic Emulsion R-1 for
Water-Based Intermediate Coating Material
First, 31.5 parts by mass of 2-ethylhexyl acrylate, 78.8 parts by
mass of butyl methacrylate, 52.9 parts by mass of styrene, 72.5
parts by mass of 4-hydroxybutyl acrylate, 16.4 parts by mass of
acrylic acid, 63.0 parts by mass of methyl methacrylate, 3.2 parts
by mass of n-dodecyl mercaptan, 119 parts by mass of ion-exchanged
water, and 17.5 parts by mass of LATEMUL (PD-104) were mixed, and
emulsified by stirring with a mixer. Thus, a monomer pre-emulsion
was prepared.
Next, into an ordinary reaction vessel for producing an acrylic
resin emulsion equipped with a stirrer, a thermometer, a dropping
funnel, a reflux condenser, a nitrogen inlet tube, and the like,
280 parts by mass of ion-exchanged water, 3.5 parts by mass of
LATEMUL PD-104 (manufactured by Kao Chemicals), and an aqueous APS
solution (obtained by mixing 0.7 parts by mass of ammonium
persulfate APS (manufactured by Aldrich), which was a
polymerization initiator, and 7 parts by mass of water with
stirring) were introduced, and heated to 80.degree. C. with
stirring. Subsequently, to this solution in the reaction vessel, 5%
by mass of the total amount of the monomer pre-emulsion was added,
and the mixture was held at 80.degree. C. for 10 minutes. After
that, the remainder of the monomer pre-emulsion was added dropwise
into the reaction vessel over 3 hours with stirring. After
completion of the dropwise addition, the reaction was further
allowed to proceed by continuing the stirring at 80.degree. C. for
1 hour. After that, 322 parts by mass of ion-exchanged water was
added thereto, and the mixture was cooled to room temperature.
After the cooling, 40.5 parts by mass of an aqueous 50% by mass
dimethylethanolamine solution was added, followed by stirring for
10 minutes. Thus, an acrylic emulsion R-1 having a hydroxyl value
of 90 and a non-volatile content of 29% by mass was obtained.
(Synthesis Example 2) Preparation of Acrylic Emulsion R-2 for
Water-Based Coating Material
First, 31.5 parts by mass of 2-ethylhexyl acrylate, 78.8 parts by
mass of butyl methacrylate, 37.8 parts by mass of butyl acrylate,
63.0 parts by mass of 2-hydroxyethyl methacrylate, 16.4 parts by
mass of acrylic acid, 87.6 parts by mass of styrene, 3.2 parts by
mass of n-dodecylmercaptan, 119 parts by mass of ion-exchanged
water, and 17.5 parts by mass of LATEMUL (PD-104) were mixed, and
emulsified by stirring with a mixer. Thus, a monomer pre-emulsion
was prepared.
Next, into an ordinary reaction vessel for producing an acrylic
resin emulsion equipped with a stirrer, a thermometer, a dropping
funnel, a reflux condenser, a nitrogen inlet tube, and the like,
280 parts by mass of ion-exchanged water, 3.5 parts by mass of
LATEMUL PD-104 (manufactured by Kao Chemicals), and an aqueous APS
solution (obtained by mixing 0.7 parts by mass of ammonium
persulfate APS (manufactured by Aldrich), which was a
polymerization initiator, and 7 parts by mass of water with
stirring) were introduced, and heated to 80.degree. C. with
stirring. Subsequently, to this solution in the reaction vessel, 5%
by mass of the total amount of the monomer pre-emulsion was added,
and the mixture was held at 80.degree. C. for 10 minutes. After
that, the remainder of the monomer pre-emulsion was added dropwise
into the reaction vessel over 3 hours with stirring. After
completion of the dropwise addition, the reaction was further
allowed to proceed by continuing the stirring at 80.degree. C. for
1 hour. After that, 322 parts by mass of ion-exchanged water was
added thereto, and the mixture was cooled to room temperature.
After the cooling, 40.5 parts by mass of an aqueous 50% by mass
dimethylethanolamine solution was added, followed by stirring for
10 minutes. Thus, an acrylic emulsion R-2 having a hydroxyl value
of 86 and a non-volatile content of 29% by mass was obtained.
(Synthesis Example 3) Preparation of Acrylic Resin R-3 for
Solvent-Based Clear Coating Material
First, 235 parts by mass of Solvesso 100 was introduced into an
ordinary reaction vessel for producing an acrylic resin equipped
with a stirrer, a thermometer, a dropping funnel, a reflux
condenser, a nitrogen inlet tube, and the like, and the temperature
was raised to 130.degree. C. with stirring.
Next, a mixture of 95 parts by mass of 2-ethylhexyl acrylate, 120
parts by mass of 2-hydroxyethyl methacrylate, 150 parts by mass of
styrene, 135 parts by mass of glycidyl methacrylate, and 40 parts
by mass of a polymerization initiator ("PERCURE O" manufactured by
NOF CORPORATION) was prepared, and the mixture was added dropwise
to the reaction vessel with stirring over 3 hours. After completion
of the dropwise addition, the reaction was allowed to proceed by
continuing the stirring at 130.degree. C. for 1 hour. After that,
10 parts by mass of PERCURE O was added, and the reaction was
allowed to proceed by further continuing the stirring at
130.degree. C. for 2 hours, followed by cooling to room
temperature. Thus, an acrylic resin R-3 having a hydroxyl value of
94, an epoxy value of 107, and a non-volatile content of 70% by
mass was obtained.
(Synthesis Example 4) Preparation of Acrylic Resin R-4 for
Solvent-Based Clear Coating Material
First, 310 part s by mass of Solves so 100 was introduced into an
ordinary reaction vessel for producing an acrylic resin equipped
with a stirrer, a thermometer, a dropping funnel, a reflux
condenser, a nitrogen inlet tube, and the like, and the temperature
was raised to 130.degree. C. with stirring.
Next, a mixture of 125 parts by mass of butyl methacrylate, 225
parts by mass of 2-ethylhexyl methacrylate, 150 parts by mass of
maleic anhydride, 50 parts by mass of Solvesso 100, and 100 parts
by mass of PERCURE 0 (polymerization initiator manufactured by NOF
CORPORATION) was prepared, and the mixture was added dropwise into
the reaction vessel with stirring over 3 hours. After completion of
the dropwise addition, the reaction was allowed to proceed by
continuing the stirring at 130.degree. C. for 1 hour. After that,
10 parts by mass of a polymerization initiator ("PERCURE O"
manufactured by NOF CORPORATION) was added, and the react ion was
al lowed to proceed by further continuing the stirring at
130.degree. C. for 2 hours, followed by cooling to 60.degree. C.
After the cooling, 4.6 parts by mass of triethylamine and 73.5
parts by mass of methanol were added, and the reaction was allowed
to proceed by continuing the stirring at 60.degree. C. for 12
hours, followed by cooling to room temperature. Thus, an acrylic
resin R-4 having an acid number of 172 and a non-volatile content
of 61% by mass was obtained.
(Synthesis Example 5) Preparation of Acrylic Resin R-5 for
Solvent-Based Clear Coating Material
First, 195 parts by mass of Solvesso 100 and 65 parts by mass of
butyl acetate were introduced into an ordinary reaction vessel for
producing an acrylic resin equipped with a stirrer, a thermometer,
a dropping funnel, a reflux condenser, a nitrogen inlet tube, and
the like, and the temperature was raised to 130.degree. C. with
stirring.
Next, a mixture of 162.5 parts by mass of butyl methacrylate, 149.5
parts by mass of 4-hydroxybutyl acrylate, 78 parts by mass of
styrene, 260 parts by mass of isobornyl acrylate, 52 parts by mass
of PERCURE O (polymerization initiator manufactured by NOF
CORPORATION) was prepared, and the mixture was added dropwise to
the reaction vessel with stirring over 3 hours. After completion of
the dropwise addition, the reaction was allowed to proceed by
continuing the stirring at 130.degree. C. for 1 hour. After that,
13 parts by mass of a polymerization initiator ("PERCURE O"
manufactured by NOF CORPORATION) was added, and the react ion was
al lowed to proceed by further continuing the stirring at
130.degree. C. for 2 hours. Then, 75 parts by mass of butyl acetate
was added, followed by cooling to room temperature. Thus, an
acrylic resin R-5 having a hydroxyl value of 90 and a non-volatile
content of 65% by mass was obtained.
(Preparation Example 1) Preparation of Colored Pigment Paste
Into a container, 450 parts of ion-exchanged water, 50 parts of a
wetting and dispersing agent ("Disperbyk-180" manufactured by
Byk-Chemie), 495 parts of rutile titanium oxide ("CR-90"
manufactured by Ishihara Sangyo Kaisha, Ltd.), and 5 parts of
carbon black ("MA-100" manufactured by Mitsubishi Chemical
Corporation) were introduced, and preliminarily mixed for 10
minutes. Then, glass beads (particle diameter: 1.6 mm) in a volume
which was equal to the volume of the materials introduced were
added, and the materials were dispersed with a desktop sand mill
for 1 hour. The grain size measured with a grind gauge was 5 .mu.m
or less at the completion of the dispersing.
(Preparation Example 2) Preparation of Water-Based Intermediate
Coating Material P-1
Into a container, 244.4 parts by mass of the acrylic emulsion R-1
for water-based intermediate coating material obtained in Synthesis
Example 1 was introduced. To this container, 27.9 parts by mass of
a hydrophilic polyisocyanate ("DURANATE WB40-100" manufactured by
Asahi Kasei Chemicals Corporation) and 15 parts by mass of butyl
glycol were added with stirring, followed by stirring for 5
minutes. Further, 6.7 parts by mass of an alkali thicker ("Viscalex
HV30" manufactured by Ciba Specialty Chemicals), 1.0 parts by mass
of dimethylethanolamine, 2.5 parts of BYK-346 (manufactured by
Byk-Chemie), and 142.3 parts by mass of the colored pigment paste
obtained in Preparation Example 1 were added. Thus, a water-based
intermediate coating material P-1 having a non-volatile content of
39.3% by mass was obtained. The water-based intermediate coating
material P-1 had a shrinkage ratio .omega.' of 0.8%.
(Preparation Example 3) Preparation of Water-Based Intermediate
Coating Material P-2
A water-based intermediate coating material P-2 was obtained in the
same manner as in Preparation Example 2, except that the amount of
the acrylic emulsion R-1 for water-based intermediate coating
material obtained in Synthesis Example 1 was changed to 313.6 parts
by mass, and 9.4 parts by mass of a methylated melamine resin
("CYMEL 325" manufactured by Nihon Cytec Industries Inc.) was used
instead of DURANATE WB40-100. The water-based intermediate coating
material P-2 had a non-volatile content of 35.3% by mass and a
shrinkage ratio .omega.' of 1.9%.
(Preparation Example 4) Preparation of Water-Based Intermediate
Coating Material P-3
A water-based intermediate coating material P-3 was obtained in the
same manner as in Preparation Example 2, except that the amount of
the acrylic emulsion R-1 for water-based intermediate coating
material obtained in Synthesis Example 1 was changed to 288.1 parts
by mass, and 18.8 parts by mass of a methylated melamine resin
("CYMEL 325" manufactured by Nihon Cytec Industries Inc.) was used
instead of DURANATE WB40-100. This water-based intermediate coating
material P-3 had a non-volatile content of 36.5% by mass and a
shrinkage ratio .omega.' of 2.7%.
(Preparation Example 5) Preparation of Water-Based Intermediate
Coating Material P-4
A water-based intermediate coating material P-4 was obtained in the
same manner as in Preparation Example 2, except that the amount of
the acrylic emulsion R-1 for water-based intermediate coating
material obtained in Synthesis Example 1 was changed to 237.3 parts
by mass, 37.5 parts by mass of a methylated melamine resin ("CYMEL
325" manufactured by Nihon Cytec Industries Inc.) was used instead
of DURANATEWB40-100, and the amount of the colored pigment paste
added was changed to 203.3 parts by mass. This water-based
intermediate coating material P-4 had a non-volatile content of
40.3% by mass and a shrinkage ratio .omega.' of 3.3%.
(Preparation Example 6) Preparation of Water-Based Intermediate
Coating Material P-5
A water-based intermediate coating material P-5 was obtained in the
same manner as in Preparation Example 2, except that the amount of
the acrylic emulsion R-1 for water-based intermediate coating
material obtained in Synthesis Example 1 was changed to 237.3 parts
by mass, and 37.5 parts by mass of a methylated melamine resin
("CYMEL 325" manufactured by Nihon Cytec Industries Inc.) was used
instead of DURANATE WB40-100. This water-based intermediate coating
material P-5 had a non-volatile content of 39.1% by mass and a
shrinkage ratio .omega.' of 3.8%.
(Preparation Example 7) Preparation of Water-Based Intermediate
Coating Material P-6
A water-based intermediate coating material P-6 was obtained in the
same manner as in Preparation Example 2, except that the amount of
the acrylic emulsion R-1 for water-based intermediate coating
material obtained in Synthesis Example 1 was changed to 237.3 parts
by mass, 37.5 parts by mass of a methylated melamine resin ("CYMEL
325" manufactured by Nihon Cytec Industries Inc.) was used instead
of DURANATEWB40-100, and the amount of the colored pigment paste
added was changed to 81.3 parts by mass. This water-based
intermediate coating material P-6 had a non-volatile content of
37.5% by mass and a shrinkage ratio .omega.' of 4.4%.
(Preparation Example 8) Preparation of Water-Based Intermediate
Coating Material P-7
A water-based intermediate coating material P-7 was obtained in the
same manner as in Preparation Example 2, except that the amount of
the acrylic emulsion R-1 for water-based intermediate coating
material obtained in Synthesis Example 1 was changed to 203.4 parts
by mass, and 50.0 parts by mass of a methylated melamine resin
("CYMEL 325" manufactured by Nihon Cytec Industries Inc.) was used
instead of DURANATE WB40-100. This water-based intermediate coating
material P-7 had a non-volatile content of 41.1% by mass and a
shrinkage ratio .omega.' of 4.5%.
(Preparation Example 9) Preparation of Water-Based Base Coating
Material B-1
Into a container, 195.5 parts by mass of the acrylic emulsion R-2
for water-based coating material obtained in Synthesis Example 2
was introduced. To this acrylic emulsion, 22.3 parts by mass of a
hydrophilic polyisocyanate ("DURANATE WB40-100" manufactured by
Asahi Kasei Chemicals Corporation), 120 parts by mass of
ion-exchanged water, and 24 parts by mass of butyl glycol were
added with stirring, followed by stirring for 5 minutes. Further,
9.3 parts by mass of an alkali thicker ("Viscalex HV30"
manufactured by Ciba Specialty Chemicals), 3.2 parts by mass of
dimethylethanolamine, and 5.0 parts by mass of SURFYNOL 104DPM
(manufactured by Nissin Chemical Industry Co., Ltd) were added.
Thus, a water-based resin liquid was obtained.
Meanwhile, into another container, 24 parts by mass of butyl glycol
and 30 parts by mass of an aluminum paste ("Hydrolan 2156"
manufactured by ECKART) were added, followed by stirring for 1
hour. Thus, an aluminum paste solution was obtained.
Next, to 379.3 parts by mass of the water-based resin solution,
52.9 parts by mass of this aluminum paste solution was added with
stirring, and further the mixture was stirred for 1 hour. Thus, a
water-based base coating material B-1 having a non-volatile content
of 23.7% by mass was obtained. This water-based base coating
material B-1 had a shrinkage ratio .omega.' of 0.5%.
(Preparation Example 10) Preparation of Water-Based Base Coating
Material B-2
A water-based base coating material B-2 was obtained in the same
manner as in Preparation Example 9, except that the amount of
introduction of the acrylic emulsion R-2 for water-based coating
material obtained in Synthesis Example 2 was changed to 250.8 parts
by mass, and 7.5 parts by mass of a methylated melamine resin
("CYMEL 325" manufactured by Nihon Cytec Industries Inc.) was used
instead of DURANATE WB40-100. This water-based base coating
material B-2 had a non-volatile content of 21.7% by mass and a
shrinkage ratio .omega.' of 2.0%.
(Preparation Example 11) Preparation of Water-Based Base Coating
Material B-3
A water-based base coating material B-3 was obtained in the same
manner as in Preparation Example 9, except that the amount of
introduction of the acrylic emulsion R-2 for water-based coating
material obtained in Synthesis Example 2 was changed to 230.5 parts
by mass, and the amount of CYMEL 325 added was changed to 15 parts
by mass. This water-based base coating material B-3 had a
non-volatile content of 22.3% by mass and a shrinkage ratio
.omega.' of 2.6%.
(Preparation Example 12) Preparation of Water-Based Base Coating
Material B-4
A water-based base coating material B-4 was obtained in the same
manner as in Preparation Example 9, except that the amount of
introduction of the acrylic emulsion R-2 for water-based coating
material obtained in Synthesis Example 2 was changed to 189.8 parts
by mass, and the amount of CYMEL 325 introduced was changed to 30.0
parts by mass. This water-based base coating material B-4 had a
non-volatile content of 23.6% by mass and a shrinkage ratio
.omega.' of 3.2%.
(Preparation Example 13) Preparation of Solvent-Based Clear Coating
Material C-1
Into a container, 443.3 parts by mass of the acrylic resin R-3 for
solvent-based clear coating material obtained in Synthesis Example
3, 300.3 parts by mass of the acrylic resin R-4 for solvent-based
clear coating material obtained in Synthesis Example 4, 123.8 parts
by mass of n-butanol, 24.8 parts by mass of Solvesso 100, 14.9
parts by mass of xylene, 39.6 parts by mass of
2-methoxy-1-propanol, 9.9 parts by mass of TINUVIN 123
(manufactured by BASF), 9.9 parts by mass of TINUVIN 384-2
(manufactured by BASF), and 9.9 parts by mass of a tributylammonium
bromide solution (a mixture of 0.9 parts by mass of
tributylammonium bromide and 9 parts by mass of n-butanol) were
introduced. To this mixture, 2.8 parts by mass of BYK-370
(manufactured by BYK-Chmie), 5.2 parts by mass of BYK-306
(manufactured by BYK-Chmie), 5.0 parts by mass of DISPARLON NSH8430
(manufactured by Kusumoto Chemicals, Ltd.), and 1.2 parts by mass
of DISPARLON OX883 (manufactured by Kusumoto Chemicals, Ltd.) were
added with stirring, followed by stirring for further 10 minutes.
Thus, an acid-epoxy curing solvent-based clear coating material C-1
having a non-volatile content of 52% was obtained. This
solvent-based clear coating material C-1 had a shrinkage ratio
.omega.' of 1.1%.
(Preparation Example 14) Preparation of Solvent-Based Clear Coating
Material C-2
Into a container, 759.3 parts by mass of the acrylic resin R-5 for
solvent-based clear coating material obtained in Synthesis Example
5, 197.4 parts by mass of butyl acetate, 9.9 parts by mass of
TINUVIN 123 (manufactured by BASF), and 9.9 parts by mass of
TINUVIN 384-Z (manufactured by BASF) were introduced. To this
mixture, 2.8 parts by mass of BYK-370 (manufactured by BYK-Chmie),
5.1 parts by mass of BYK-306 (manufactured by BYK-Chmie), 9.5 parts
by mass of BYK-392 (manufactured by BYK-Chmie), 4.9 parts by mass
of DISPARLONNSH8430 (Kusumoto Chemicals, Ltd.), 1.2 parts by mass
of DISPARLON OX883 (manufactured by Kusumoto Chemicals, Ltd.), and
175 parts by mass of a polyisocyanate ("DURANATE TPA-100"
manufactured by Asahi Kasei Chemicals Corporation) were added with
stirring, followed by stirring for further 10 minutes. Thus, an
isocyanate-curing solvent-based clear coating material C-2 having a
non-volatile content of 59% was obtained. This solvent-based clear
coating material C-2 had a shrinkage ratio .omega.' of 0.2%.
Example 1
On a surface of a steel plate (manufactured by Japan Route Service
K. K.) subjected to electrodeposition, the water-based intermediate
coating material P-1 (shrinkage ratio .omega.':0.8%) obtained in
Preparation Example 2 was applied in a film thickness which became
20 .mu.m after baking. Next, the steel plate was allowed to stand
(flashed) at room temperature for 4 minutes. Then, the water-based
base coating material B-1 (shrinkage ratio .omega.':0.5%) obtained
in Preparation Example 9 was applied in a film thickness which
became 15 .mu.m after baking. Then, water, the organic solvent, and
the like were evaporated by heating at 80.degree. C. for 3 minutes.
Subsequently, on this layer of the water-based base coating
material B-1, the solvent-based clear coating material C-2
(shrinkage ratio .omega.':0.2%) obtained in Preparation Example 14
was applied in a film thickness which became 35 .mu.m after baking.
Thus, an uncured laminated coating film was obtained in which the
water-based intermediate coating material P-1, the water-based base
coating material B-1, and the solvent-based clear coating material
C-2 were applied using a wet-on-wet technique.
After this uncured laminated coating film was allowed to stand
(flashed) at room temperature for 10 minutes, the uncured laminated
coating film was subjected to a heat treatment (baking treatment)
at 140.degree. C. for 30 minutes to cause the curing reaction.
Thus, the layers were cured, and a laminated coating film was
obtained.
The obtained laminated coating film was measured for wave scan
values [du (wavelength<0.1 mm), Wa (wavelength<0.3 mm), Wb
(wavelength: 0.3 to 1 mm), Wc (wavelength: 1 to 3 mm), Wd
(wavelength: 3 to 10 mm), and We (wavelength: 10 to 30 mm)] by
using a wave scan ("Wave-Scan Dual" manufactured by BYK-Gardner).
Table 1 shows the results. Regarding these wave scan values, a
smaller value means that the surface of the upper layer has less
unevenness corresponding to the wavelengths, and is better in
appearance qualities. Here, a smaller du or Wa means better gloss,
and a smaller Wd or We means better surface texture. The required
appearance quality is 15 or less in terms of Wa.
In addition, the absolute value |.DELTA..omega..sub.A'| of the
difference between the shrinkage ratio of the water-based
intermediate coating material P-1 (lower layer-coating material) at
the late stage of the baking in the baking step and the shrinkage
ratio of the water-based base coating material B-1 (intermediate
layer-coating material) at the late stage of the baking in the
baking step was 0.3%, and the absolute value
|.DELTA..omega..sub.B'| of the difference between the shrinkage
ratio of the water-based base coating material B-1 (intermediate
layer-coating material) at the late stage of the baking in the
baking step and the shrinkage ratio of the solvent-based clear
coating material C-2 (upper layer-coating material) at the late
stage of the baking in the baking step was 0.3%. Accordingly, the
sum of the absolute value (|.DELTA..omega..sub.A'|) of the
difference in shrinkage ratio between the lower layer-coating
material and the intermediate layer-coating material at the late
stage of the baking in the baking step and the absolute value
(|.DELTA..omega..sub.B'|) of the difference in shrinkage ratio
between the intermediate layer-coating material and the upper
layer-coating material at the late stage of the baking in the
baking step
(|.DELTA..omega.'|=|.DELTA..omega..sub.A'|+|.DELTA..omega..sub.B'|)
was 0.6%.
Example 2
A laminated coating film was obtained in the same manner as in
Example 1, except that the water-based intermediate coating
material P-3 (shrinkage ratio .omega.':2.7%) obtained in
Preparation Example 4 was used instead of the water-based
intermediate coating material P-1, the water-based base coating
material B-2 (shrinkage ratio .omega.':2.0%) obtained in
Preparation Example 10 was used instead of the water-based base
coating material B-1, and the solvent-based clear coating material
C-1 (shrinkage ratio .omega.':1.1%) obtained in Preparation Example
13 was used instead of the solvent-based clear coating material
C-2. The obtained laminated coating film was measured for du and Wa
to We in the same manner as in Example 1. Table 1 shows the
results. Note that the absolute value |.DELTA..omega..sub.A'| of
the difference between the shrinkage ratio of the water-based
intermediate coating material P-3 (lower layer-coating material)
and the shrinkage ratio of the water-based base coating material
B-2 (intermediate layer-coating material) at the late stage of the
baking in the baking step was 0.7%, and the absolute value
|.DELTA..omega..sub.B'| of the difference between the shrinkage
ratio of the water-based base coating material B-2 (intermediate
layer-coating material) and the shrinkage ratio of the
solvent-based clear coating material C-1 (upper layer-coating
material) at the late stage of the baking in the baking step was
0.9%. Accordingly,
|.DELTA..omega.'|=|.DELTA..omega..sub.A'|+|.DELTA..omega..sub.B'|=1.6%.
Example 3
A laminated coating film was obtained in the same manner as in
Example 1, except that the water-based intermediate coating
material P-2 (shrinkage ratio .omega.':1.9%) obtained in
Preparation Example 3 was used instead of the water-based
intermediate coating material P-1, and the water-based base coating
material B-2 (shrinkage ratio .omega.':2.0%) obtained in
Preparation Example 10 was used instead of the water-based base
coating material B-1. The obtained laminated coating film was
measured for du and Wa to We in the same manner as in Example 1.
Table 1 shows the results. Note that the absolute value
|.DELTA..omega..sub.A'| of the difference between the shrinkage
ratio of the water-based intermediate coating material P-2 (lower
layer-coating material) and the shrinkage ratio of the water-based
base coating material B-2 (intermediate layer-coating material) at
the late stage of the baking in the baking step was 0.1%, and the
absolute value |.DELTA..omega..sub.B'| of the difference between
the shrinkage ratio of the water-based base coating material B-2
(intermediate layer-coating material) and the shrinkage ratio of
the solvent-based clear coating material C-2 (upper layer-coating
material) at the late stage of the baking in the baking step was
1.8%. Accordingly,
|.DELTA..omega.'|=|.DELTA..omega..sub.A'+|.DELTA..omega..sub.B'|=1.9%.
Example 4
A laminated coating film was obtained in the same manner as in
Example 1, except that the water-based intermediate coating
material P-4 (shrinkage ratio .omega.':3.3%) obtained in
Preparation Example 5 was used instead of the water-based
intermediate coating material P-1, the water-based base coating
material B-4 (shrinkage ratio .omega.':3.2%) obtained in
Preparation Example 12 was used instead of the water-based base
coating material B-1, and the solvent-based clear coating material
C-1 (shrinkage ratio .omega.':1.1%) obtained in Preparation Example
13 was used instead of the solvent-based clear coating material
C-2. The obtained laminated coating film was measured for du and Wa
to We in the same manner as in Example 1. Table 1 shows the
results. Note that the absolute value |.DELTA..omega..sub.A'| of
the difference between the shrinkage ratio of the water-based
intermediate coating material P-4 (lower layer-coating material)
and the shrinkage ratio of the water-based base coating material
B-4 (intermediate layer-coating material) at the late stage of the
baking in the baking step was 0.1%, and the absolute value
|.DELTA..omega..sub.B'| of the difference between the shrinkage
ratio of the water-based base coating material B-4 (intermediate
layer-coating material) and the shrinkage ratio of the
solvent-based clear coating material C-1 (upper layer-coating
material) at the late stage of the baking in the baking step was
2.1%. Accordingly,
|.DELTA..omega.'|=|.DELTA..omega..sub.A'|+|.DELTA..omega..sub.B'|=2.2%.
Example 5
A laminated coating film was obtained in the same manner as in
Example 1, except that the water-based intermediate coating
material P-3 (shrinkage ratio .omega.':2.7%) obtained in
Preparation Example 4 was used instead of the water-based
intermediate coating material P-1, and the water-based base coating
material B-2 (shrinkage ratio .omega.':2.0%) obtained in
Preparation Example 10 was used instead of the water-based base
coating material B-1. The obtained laminated coating film was
measured for du and Wa to We in the same manner as in Example 1.
Table 1 shows the results. Note that the absolute value
|.DELTA..omega..sub.A'| of the difference between the shrinkage
ratio of the water-based intermediate coating material P-3 (lower
layer-coating material) and the shrinkage ratio of the water-based
base coating material B-1 (intermediate layer-coating material) at
the late stage of the baking in the baking step was 0.7%, and the
absolute value |.DELTA..omega..sub.B'| of the difference between
the shrinkage ratio of the water-based base coating material B-1
(intermediate layer-coating material) and the shrinkage ratio of
the solvent-based clear coating material C-2 (upper layer-coating
material) at the late stage of the baking in the baking step was
1.8%. Accordingly,
|.DELTA..omega.'|=|.DELTA..omega..sub.A'+|.DELTA..omega..sub.B'|=2.5%.
Example 6
A laminated coating film was obtained in the same manner as in
Example 1, except that the water-based intermediate coating
material P-5 (shrinkage ratio .OMEGA.':3.8%) obtained in
Preparation Example 6 was used instead of the water-based
intermediate coating material P-1, the water-based base coating
material B-3 (shrinkage ratio .omega.':2.6%) obtained in
Preparation Example 11 was used instead of the water-based base
coating material B-1, and the solvent-based clear coating material
C-1 (shrinkage ratio .omega.':1.1%) obtained in Preparation Example
13 was used instead of the solvent-based clear coating material
C-2. The obtained laminated coating film was measured for du and Wa
to We in the same manner as in Example 1. Table 1 shows the
results. Note that the absolute value |.DELTA..omega..sub.A'| of
the difference between the shrinkage ratio of the water-based
intermediate coating material P-5 (lower layer-coating material)
and the shrinkage ratio of the water-based base coating material
B-3 (intermediate layer-coating material) at the late stage of the
baking in the baking step was 1.2%, and the absolute value
|.DELTA..omega..sub.B'| of the difference between the shrinkage
ratio of the water-based base coating material B-3 (intermediate
layer-coating material) and the shrinkage ratio of the
solvent-based clear coating material C-1 (upper layer-coating
material) at the late stage of the baking in the baking step was
1.5%. Accordingly,
|.DELTA..omega.'|=|.DELTA..omega..sub.A'|+|.DELTA..omega..sub.B'|=2.7%.
Comparative Example 1
A laminated coating film for comparison was obtained in the same
manner as in Example 1, except that the water-based intermediate
coating material P-6 (shrinkage ratio .omega.':4.4%) obtained in
Preparation Example 7 was used instead of the water-based
intermediate coating material P-1, the water-based base coating
material B-4 (shrinkage ratio .omega.':3.2%) obtained in
Preparation Example 12 was used instead of the water-based base
coating material B-1, and the solvent-based clear coating material
C-1 (shrinkage ratio .omega.':1.1%) obtained in Preparation Example
13 was used instead of the solvent-based clear coating material
C-2. The obtained laminated coating film for comparison was
measured for du and Wa to We in the same manner as in Example 1.
Table 1 shows the results. Note that the absolute value
|.DELTA..omega..sub.A'| of the difference between the shrinkage
ratio of the water-based intermediate coating material P-6 (lower
layer-coating material) and the shrinkage ratio of the water-based
base coating material B-4 (intermediate layer-coating material) at
the late stage of the baking in the baking step was 1.2%, and the
absolute value |.DELTA..omega..sub.B'| of the difference between
the shrinkage ratio of the water-based base coating material B-4
(intermediate layer-coating material) and the shrinkage ratio of
the solvent-based clear coating material C-1 (upper layer-coating
material) at the late stage of the baking in the baking step was
2.1%. Accordingly,
|.DELTA..omega.'|=|.DELTA..omega..sub.A'|+|.DELTA..omega..sub.B'|=3.3%.
Comparative Example 2
A laminated coating film for comparison was obtained in the same
manner as in Example 1, except that the water-based intermediate
coating material P-7 (shrinkage ratio .omega.':4.5%) obtained in
Preparation Example 8 was used instead of the water-based
intermediate coating material P-1, the water-based base coating
material B-4 (shrinkage ratio .omega.':3.2%) obtained in
Preparation Example 12 was used instead of the water-based base
coating material B-1, and the solvent-based clear coating material
C-1 (shrinkage ratio .omega.':1.1%) obtained in Preparation Example
13 was used instead of the solvent-based clear coating material
C-2. The obtained laminated coating film for comparison was
measured for du and Wa to We in the same manner as in Example 1.
Table 1 shows the results. Note that the absolute value
|.DELTA..omega..sub.A'| of the difference between the shrinkage
ratio of the water-based intermediate coating material P-7 (lower
layer-coating material) and the shrinkage ratio of the water-based
base coating material B-4 (intermediate layer-coating material) at
the late stage of the baking in the baking step was 1.3%, and the
absolute value |.DELTA..omega..sub.B'| of the difference between
the shrinkage ratio of the water-based base coating material B-4
(intermediate layer-coating material) and the shrinkage ratio of
the solvent-based clear coating material C-1 (upper layer-coating
material) at the late stage of the baking in the baking step was
2.1%. Accordingly,
|.DELTA..omega.'|=|.DELTA..omega..sub.A'|+|.DELTA..omega..sub.B'|=3.4%.
Comparative Example 3
A laminated coating film for comparison was obtained in the same
manner as in Example 1, except that the water-based intermediate
coating material P-6 (shrinkage ratio .omega.':4.4%) obtained in
Preparation Example 8 was used instead of the water-based
intermediate coating material P-1, and the water-based base coating
material B-4 (shrinkage ratio .omega.':3.2%) obtained in
Preparation Example 12 was used instead of the water-based base
coating material B-1. The obtained laminated coating film for
comparison was measured for du and Wa to We in the same manner as
in Example 1. Table 1 shows the results. Note that the absolute
value |.DELTA..omega..sub.A'| of the difference between the
shrinkage ratio of the water-based intermediate coating material
P-6 (lower layer-coating material) and the shrinkage ratio of the
water-based base coating material B-4 (intermediate layer-coating
material) at the late stage of the baking in the baking step was
1.2%, and the absolute value |.DELTA..omega..sub.B'| of the
difference between the shrinkage ratio of the water-based base
coating material B-4 (intermediate layer-coating material) and the
shrinkage ratio of the solvent-based clear coating material C-2
(upper layer-coating material) at the late stage of the baking in
the baking step was 3.0%. Accordingly,
|.DELTA..omega.'|=|.DELTA..omega..sub.A'|+|.DELTA..omega..sub.B'|=4.2%.
TABLE-US-00001 TABLE 1 Intermediate Base Clear coating coating
coating material material material |.DELTA..omega..sub.A`|
|.DELTA..omega..sub.B`- | |.DELTA..omega.`| du Wa Wb Wc Wd We
Example 1 P-1 B-1 C-2 0.3 0.3 0.6 30.8 12.4 15.3 8.4 8.1 6.0
Example 2 P-3 B-2 C-1 0.7 0.9 1.6 31.4 14.5 19.7 9.2 9.8 6.4
Example 3 P-2 B-2 C-2 0.1 1.8 1.9 28.5 10.6 17.9 7.9 6.0 6.3
Example 4 P-4 B-4 C-1 0.1 2.1 2.2 31.6 14.3 20.0 8.8 7.0 5.8
Example 5 P-3 B-2 C-2 0.7 1.8 2.5 29.4 11.6 19.5 8.2 8.4 5.9
Example 6 P-5 B-3 C-1 1.2 1.5 2.7 30.7 14.1 22.0 10.8 10.9 8.2
Comp. Ex. 1 P-6 B-4 C-1 1.2 2.1 3.3 34.0 22.2 26.9 12.4 13.4 12.2
Comp. Ex. 2 P-7 B-4 C-1 1.3 2.1 3.4 35.5 22.0 26.6 12.2 14.5 13.5
Comp. Ex. 3 P-6 B-4 C-2 1.2 3.0 4.2 33.1 20.3 27.3 14.6 13.8
11.3
Here, the laminated coating films (Examples 1 to 6) were formed in
such a manner that the uncured laminated coating film was obtained
by using thermosetting coating materials for all of the lower
layer, the intermediate layer, and the upper layer, and applying
the thermosetting coating materials using a wet-on-wet technique,
and the uncured laminated coating film was then subjected to the
baking treatment, with the sum
(|.DELTA..omega.'|=|.DELTA..omega..sub.A'|+|.DELTA..omega..sub.B'|)
of the absolute value (|.DELTA..omega..sub.A'|) of the difference
in shrinkage ratio between the water-based intermediate coating
material (lower layer-coating material) and the water-based base
coating material (intermediate layer-coating material) at the late
stage of the baking in the baking step and the absolute value
(|.DELTA..omega..sub.B'|) of the difference in shrinkage ratio
between the water-based base coating material (intermediate
layer-coating material) and the solvent-based clear coating
material (upper layer-coating material) at the late stage of the
baking in the baking step being within the range of 3.0 or smaller,
as described in the present invention. Meanwhile, the conventional
laminated coating films (Comparative Examples 1 to 3) had the
absolute values |.DELTA..omega.'| exceeding 3.0. As is apparent
from the results shown in Table 1, it was found that the laminated
coating films (Examples 1 to 6) had smaller du and Wa to Wd values
than the conventional laminated coating films (Comparative Examples
1 to 3), and were very excellent in appearance qualities.
Specifically, there was a tendency that the du and Wa to We values
decreased with the decrease in |.DELTA..omega.'|, and the Wa of
each of the coating films in which the lower layer-coating
material, the intermediate layer-coating material, and the upper
layer-coating material were applied using a wet-on-wet technique
with the |.DELTA..omega.'| being 3.0% or smaller as described in
the present invention was 15 or lower, and satisfied the required
appearance quality. In contrast, it was found that the Wa of each
of the laminated coating films of Comparative Examples (Comparative
Examples 1 to 3), in which the lower layer-coating material, the
intermediate layer-coating material, and the upper layer-coating
material were applied using a wet-on-wet technique with the
|.DELTA..omega.'| being large than 3.0%, exceeded 20, and did not
satisfy the required appearance quality.
As described above, it has been found that a laminated coating film
having very excellent appearance qualities can be obtained when
three kinds of coating materials are applied using a wet-on-wet
technique, and the sum of the absolute value of the difference in
shrinkage ratio between the lower layer-coating material and the
intermediate layer-coating material at the late stage of the baking
in the baking step and the absolute value of the difference in
shrinkage ratio between the intermediate layer-coating material and
the upper layer-coating material at the late stage of the baking in
the baking step is 3.0% or smaller.
INDUSTRIAL APPLICABILITY
As has been described above, according to the present
invention,
a laminated coating film having an upper layer in which formation
of surface unevenness is sufficiently suppressed can be obtained,
even when three kinds of coating materials are applied using a
wet-on-wet technique, and simultaneously baked to cure the layers.
This makes it possible to obtain a coated article having very
excellent appearance qualities such as surface texture (surface
smoothness) and gloss.
Accordingly, the present invention is useful as a coating method
which makes it possible to obtain a coated article having very
excellent appearance qualities, even when three kinds of coating
materials are applied using a wet-on-wet technique and then
simultaneously baked. The present invention is especially useful as
a method for coating vehicle bodies and parts for automobiles such
as passenger cars, trucks, buses, and motorcycles.
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